1. Field of the Invention
The present invention relates to a catadioptric reduction optical system suitably applied to a projection optical system for reduction projection in a projection exposure apparatus of a one-shot exposure method or a scanning exposure method, used to manufacture a semiconductor element or a liquid crystal display element in a photolithographic process and, more particularly, to a catadioptric reduction projection optical system having a magnification of about 1/4 to 1/5 with a resolution on the submicron order in the ultraviolet wavelength range.
2. Related Background Art
In fabricating semiconductor devices or liquid crystal display devices, etc. by photolithography process, the projection exposure apparatus is used for demagnifying through a projection optical system a pattern image on a reticle (or photomask, etc.) for example at a ratio of about 1/4 to 1/5 to effect exposure of the image on a wafer (or glass plate, etc.) coated with a photoresist and the like.
With the recent increase in the integration degree of semiconductor elements and the like, a higher resolution is required for a projection optical system used in a projection exposure apparatus. In order to meet this requirement, the wavelength of illumination light (exposure wavelength) for exposure must be shortened, or the numerical aperture (NA) of the projection optical system must be increased. If, however, the exposure wavelength is shortened, the types of optical glass which can be used in practice are limited because of the absorption of illumination light. In particular, as the exposure wavelength becomes 300 nm or less, only synthetic quartz and fluorite can be used in practice as glass materials.
The difference between the Abbe constants of the synthetic quartz and the fluorite is not large enough to correct chromatic aberration. For this reason, if the exposure wavelength becomes 300 nm or less, and a projection optical system is constituted by only a refracting optical system, chromatic aberration correction is very difficult to perform. In addition, since fluorite undergoes a considerable change in refractive index with a change in temperature, i.e., has poor temperature characteristics, and involves many problems in a lens polishing process, fluorite cannot be used for many portions. It is, therefore, very difficult to form a projection optical system having a required solution by using only a refracting system.
In contrast to this, attempts have been made to form a projection optical system by using only a reflecting system. In this case, however, the projection optical system increases in size and requires aspherical reflecting surfaces. It is very difficult to manufacture large, high-precision, aspherical surfaces
Under the circumstances, various techniques have been proposed to form a reduction projection optical system by using a so-called catadioptric optical system constituted by a combination of a reflecting system and a refracting system consisting of optical glass usable in relating to the exposure wavelength to be used. As an example, a reduction projection exposure apparatus including a catadioptric projection optical system having a beam splitter constituted by a cubic prism and serving to project a reticle image entirely by using a light beam near the optical axis is disclosed in, e.g., U.S. Pat. Nos. 4,953,960, 5,220,454, 5,089,913, or 5,402,267.
The present invention has as its object to provide a catadioptric reduction projection optical system which can use a beam splitting optical system smaller in size than a conventional polarizing beam splitter, can set a long optical path from a concave reflecting mirror to the image plane, can easily adjust the optical system, and has excellent imaging performance.
It is another object of the present invention to provide a catadioptric reduction projection optical system which can reduce the size of a beam splitting optical system such as a polarizing beam splitter and still has a space in which an aperture stop can be arranged.
It is still another object of the present invention to provide a catadioptric reduction projection optical system which uses a compact beam splitting optical system and can be applied to a projection optical apparatus of the scanning exposure scheme.
The catadioptric reduction projection optical system can be applied to a projection exposure apparatus of a scanning exposure method, based on use of a compact beam splitting means such as a polarizing beam splitter and the like. Besides a projection exposure apparatus of a one-shot exposure method, the catadioptric reduction projection optical system can be also applied to a recent apparatus employing a scanning exposure method such as the slit scan method or the step-and-scan method, etc. for effecting exposure while relatively scanning a reticle and a wafer to a projection optical system.
To achieve the above objects, as shown in FIGS. 1 and 2. an projection exposure apparatus of the present invention comprises at least a wafer stage 3 being movable and allowing photosensitive substrate W to be held on a main surface thereof, an illumination optical system 1 for emitting exposure light of a predetermined wavelength and transferring a predetermined pattern of a mask (reticle R) onto the substrate W, and a catadioptric reduction projection optical system 5 provided between a first surface P1 on which the mask R is disposed and a second surface P2 on which a surface of the substrate W is corresponded, for projecting an image of the pattern of the mask R onto the substrate W. The illumination optical system 1 includes an alignment optical system 110 for adjusting a relative positions between the mask R and the substrate W, and the mask R is disposed on a reticle stage 2 which is movable in parallel with respect to the main surface of the wafer stage 3. The catacioptric reduction projection optical system has a space permitting an aperture stop 6 to be set therein. The sensitive substrate W comprises a wafer 8 such as a silicon wafer or a glass plate, etc., and a photosensitive material 7 such as a photoresist and the like coating a surface of the wafer 8.
In particular, the catadioptric reduction projection optical system, as shown in FIGS. 3 and 4, includes at least a first imaging optical system having a focal length f1 (refracting lens group or first dioptric imaging optical sub-system G1(f1)) having a positive refractive power and for forming a first intermediate (i.e., primary) image 9 as a reduced image of the pattern on the object plane P1, beam splitting means 10 for splitting at least part of a light beam from the first imaging optical system, a second imaging optical system having a focal length f2 (catadioptric lens group or catadioptric imaging optical sub-system G2(f2)) including a concave reflecting mirror M1 for reflecting a light beam split by the beam splitting means, and for forming a second intermediate (i.e., secondary) image 12 as an image of the first intermediate image 9, and a third imaging optical system having a focal length f3 (refracting lens group or second dioptric imaging optical sub-system G3(f3)) for forming a third intermediate image (a final image) as an image of the second intermediate image 12 on the image plane P2 on the basis of a light beam, of a light beam from the second imaging optical system, which is split by the beam splitting means 10.
Since the first imaging optical system G1(f1) forms a first reduced intermediate image in an optical path from the image plane P1 to the concave reflecting mirror M1 (or M2), the beam splitting means can exactly carry out the splitting of a light beam from the first imaging optical system G1(f1). Since the second imaging optical system G2(f2) forms a second intermediate image in an optical path from the concave reflecting mirror M1 (or M2) to the third imaging optical system G3(f3), a smaller beam splitting means can be used in the catadioptric reduction optical system of the present invention. Additionally, as shown in FIGS. 3 and 4, the second imaging optical system G2(f2) can be used the concave reflecting mirror M1 so that the mirror M1 sandwiches the beam splitter 10 with the first imaging optical system G1(f1) and, also can be used the concave reflecting mirror M2 so that the mirror M2 sandwiches the beam splitter 10 with the third imaging optical system G3(f3).
If the beam splitting means is a prism type beam splitter 10, at least one of the first and second intermediate images 9, 12 is preferably formed in the prism type beam splitter.
If the beam splitting means is a partial reflecting mirror 13 (i.e., a turning mirror) for partially reflecting a light beam from the first imaging optical system (refracting lens group G1(f1)) as shown in FIG. 5, the second intermediate image 12 is preferably formed in an optical path form  from the first imaging optical system to the concave reflecting mirror M1 of the second imaging optical system and is located at the concave reflecting mirror side of the partial reflecting mirror 13. In other words, the intermediate image 12 is formed between the concave reflecting mirror M1 and the partial reflecting mirror 13. The partial reflecting mirror 13 is shown located off the optical axis of the catadioptric optical imaging sub-system G2(f2)), where the catadioptric optical axis intersects the optical axis of the second dioptric optical imaging sub-system G3(f3))
In addition, the following inequalities are preferably satisfied:
p1+p3 greater than 0 xe2x80x83xe2x80x83(1) 
p2 less than 0 xe2x80x83xe2x80x83(2) 
|p1+p2+p3| less than 0.1 xe2x80x83xe2x80x83(3) 
where p1, p2, and p3 are the Petzval sums of the first imaging optical system (refracting lens group G1(f2)), the second imaging optical system (catadioptric lens group G2(f2)), and the third imaging optical system (refracting lens group G3(f3)), respectively.
Further, the following relations are preferably satisfied:
0.1xe2x89xa6|xcex21|xe2x89xa61 xe2x80x83xe2x80x83(4) 
0.5xe2x89xa6|xcex22|xe2x89xa62 xe2x80x83xe2x80x83(5) 
0.25xe2x89xa6|xcex23|xe2x89xa61.5 xe2x80x83xe2x80x83(6) 
|xcex21xc2x7xcex22xc2x7xcex23|xe2x89xa61 xe2x80x83xe2x80x83(7) 
where xcex21 is the magnification between the pattern of the first surface and the first intermediate image, xcex22 is the magnification between the first intermediate image and the second intermediate image, and xcex23 is the magnification between the second intermediate image and the third intermediate image.
According to the catadioptric reduction projection optical system of the present invention, when the polarizing beam splitter 10 (PBS) is used as a beam splitting means as shown in FIGS. 3 and 4, the system is suitable for the one-shot exposure method even though the system can be applied to the scanning exposure method. In this case, a light beam incident on the second imaging optical system (catadioptric lens group G2(f2)) and a light beam reflected thereby are split by the polarizing beam splitter 10 to be guided to the subsequent optical system. In addition, the polarizing beam splitter 10 is arranged near the position where the light beam is focused as the second intermediate image 12 after the first intermediate image 9 is formed, i.e., the portion where the light beam is intensively focused. Therefore, the polarizing beam splitter 10 can be reduced in size. In addition, the blanket wafer exposure scheme can be employed unlike a so-called ring field optical system for exposing only an annular zone by using an off-axis beam.
In addition, by using a light beam from the second intermediate image 12, an image can be formed again on the second surface P2 by the third imaging optical system (refracting lens group G3(f3)). For this reason, the working distance from, e.g., a wafer placed on the second surface to the third imaging optical system (G3(f3)) can be set to be long. In addition, since an aperture stop 6 can be easily arranged in the third imaging optical system (G3(f3)), the coherent factor ("sgr" value) as the ratio between the numerical aperture of the illumination optical system and that of the projection optical system can be controlled in a wide range, thereby controlling the imaging characteristics.
Theoretically, the number of lenses of the third imaging optical system (G3(f3)) can be increased infinitely. For this reason, the numerical aperture (NA) of the projection optical system can be increased. That is, a bright optical system can be obtained.
In a general reflecting optical system, an optical path must always be deflected at a given position in the optical system. The precision in decentering the optical axis at the deflected portion is strict, posing serious problems in the manufacture of the optical system. In the present invention, however, if, for example, the optical path of a light beam from the second imaging optical system (G2(f2)) is deflected by the polarizing beam splitter 10, decentering of an optical system constituted by the first and second imaging optical systems (G1(f1), G2(f2)) and decentering of the third imaging optical system (G3) can be independently adjusted. Thereafter, a structure for combining the two optical systems at a right angle can be employed. Therefore, decentering adjustment and the like are theoretically facilitated.
With regard to this point, according to the present invention, since the polarizing beam splitter 10 is arranged near the first intermediate image 9 or the second intermediate image 12 having a relatively low decentering sensitivity, even if decentering occurs in deflecting the optical path, the influence of this decentering on the optical performance is small.
In addition, as shown in FIGS. 3 and 4, even if, for example, a wafer w is horizontally placed on the second surface P2, since, for example, a reticle on the first surface P1 and the first imaging optical system (G1(f1)) can be horizontally arranged, the overall projection optical system can be set to be lower in height than a conventional projection optical system constituted by a refracting lens system. That is, the vertical dimension can be reduced. In other words, since there is a good vertical dimension margin, the optical system can be arranged with a good margin.
In order to reduce the light amount loss in the polarizing beam splitter 10, it is preferable that a prism type beam splitter 10 be used as a polarizing beam splitter, and a xcex/4 plate 11 be inserted between the polarizing beam splitter and a concave reflecting mirror M1, as shown in, e.g., FIGS. 3 and 4. With this arrangement, most of light reflected by the concave reflecting mirror M1 is guided to the third imaging optical system (G3(f3)) via the polarizing beam splitter 10.
As shown in FIG. 5, when a partial mirror 13 is used as a beam splitting means, the basic function is almost the same as that in the case wherein the prism type beam splitter 10 is used. When the mirror 13 is used, since almost 100% of an imaging light beam can be used, occurrence of flare is suppressed. However, when the mirror 13 is used, since off-axis light offset from the optical axis is mainly used, a slit-shaped area 24 offset from the optical axis on the second surface P2 becomes an exposure field, as shown in FIGS. 5-7. Therefore, in using the mirror 13, in order to expose the pattern formed on the entire surface of a reticle R placed on the first surface P1 onto a wafer W, the reticle R and the wafer W must be scanned at a speed corresponding to the selected projection magnification. That is, exposure must be performed with the scanning exposure method.
When the small mirror 13 is used, a light beam from an annular zone offset from the axis can be used, as shown in FIGS. 8-10. In this case, the optical performance can be improved because the optical performance with respect to a portion of an image plane P2 need only be considered. Note that when the reticle R is also placed on a horizontal plane in a scheme of scanning both the reticle R and the wafer W, a mirror or the like may be arranged in the first imaging optical system (G1(f1)) to steer the optical path.
In addition, by imparting a slight field angle to, e.g., the partial reflecting mirror 13 in FIG. 5, the optical path can be split. That is, since a large field angle is not required to split the optical path, there is a good imaging performance margin as well. With regard to this point, in the conventional catadioptric projection optical system, for example, a maximum field angle of about 20xc2x0 or more is required to split the optical path. In contrast to this, a light beam incident on the mirror in the present invention exhibits a field angle of about 10xc2x0, and hence aberration correction is facilitated.
A so-called ring field optical system is known as a projection optical system for the scanning exposure method, and the ring field optical system is constructed to illuminate only an off-axis annular portion. It is, however, difficult for the ring field optical system to have a large numerical aperture, because it uses an off-axis beam. Further, because optical members in that system are not symmetric with respect to the optical axis, processing, inspection, and adjustment of the optical members are difficult, and accuracy control or accuracy maintenance is also difficult. In contrast with it, because the angle of view is not large in the present invention, the optical system is constructed in a structure with less eclipse of beam.
In the present invention, in order to improve the performance of an optical system, the Petzval sum of the overall optical system must be set to be near 0. For this purpose, inequalities (1) to (3) above are preferably satisfied.
By satisfying inequalities (1) to (3) above, the curvature of field, which is associated with the optical performance, is suppressed to improve the flatness of the image plane. The image plane is curve toward the object plane P1 in a concave form beyond the upper limit of inequality (3) (p1+p2+p3xe2x89xa70.1), and is curved toward the object plane P1 in a convex form below the lower limit of inequality (3) (p1+p2+p3xe2x89xa70.1). As a result, the imaging performance considerably deteriorates.
When part of an off-axis imaging light beam is to be used, i.e., ring field illumination is to be performed, the Petzval conditions represented by inequalities (1) to (3) above need not always be satisfied. That is, even if the image plane is curved, no problems are posed as long as the optical performance with respect to part of the image height is good.
When relations (4) to (7) associated with the first to third imaging magnifications xcex21 to xcex23 are satisfied, an optical system can be easily arranged. Below the lower limits of relations (4) to (6) above, the reduction ratio excessively increases. As a result, exposure in a wide range is difficult to perform. Beyond the upper limits of relations (4) to (6) above, the enlargement ratio excessively increases. The application of this optical system to a projection optical apparatus contradicts the essential purpose of reduction projection.
When relation (4) is satisfied, most of the reduction ratio of the overall optical system can be ensured by the first imaging optical system (G1(f1)). In this case, the prism type beam splitter 10 or the partial reflecting mirror 13, in particular, can be reduced in size.
In applying the present invention to an exposure apparatus, in order to prevent a change in magnification with variations in image plane, at which a wafer or the like is located, in the optical axis direction, a telecentric state is preferably ensured at least on the image plane side.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.